The present invention relates to copolymers, in particular suitable for use in contact lenses.
The use of synthetic hydrogels for contact lenses was first demonstrated by Wichtecte and Lim in the 1960""s. Early hydrogels employed 2-hydroxyethyl methacrylate (HEMA) as principal monomer, together with some of the homologous esters of the glycol monomethacrylate series such as diethylene glycol monomethacrylate and tetraethylene glycol monomethacrylate. It was later found that slightly crosslinked copolymers of the higher glycol monomethyacrylates and 2-hydroxyethyl methacrylate yielded transparent hydrogels that swelled in water to a higher hydration than the hydrogels of 2-hydroxyethyl methacrylate.
The water content of hydroxyalkyl methacrylate based gels can be further increased by the addition of vinyl lactams, methacrylic acids, acrylic acids, acrylamides and methacrylamides. Although the required degree of gel hydration can be achieved by the addition of anionic monomers, it is well known that these gels display high levels of protein deposition on and occasionally within the gel matrix.
It has now surprisingly been found that effective contact lens materials which have both good transparency and a high degree of water swellability are provided by copolymers which have a permanent positive charge built into them. Such polymers are formed by polymerising and crosslinking a neutral diluent monomer, for example HEMA, with a co-monomer bearing a centre of permanent positive charge. These formulations have been found to have a high level of protein resistance to tear component deposition and a reduction in lens water loss.
Accordingly, the present invention provides a crosslinked copolymer which is obtainable by polymerising a neutral diluent monomer or monomers, a monomer or monomers bearing a centre of permanent positive charge, and a bifunctional or trifunctional crosslinking agent.
The crosslinked copolymers of the present invention therefore comprise residues of a diluent monomer or monomers, a monomer or monomers bearing a centre of permanent positive charge, and a bifunctional or trifunctional crosslinking agent.
The copolymers of the invention may be xerogels which do not contain any water. Alternatively, they may be in the form of hydrogels which do contain water.
The invention also provides a process for producing such a crosslinked copolymer, a contact lens material comprising such a copolymer, a contact lens made from such a copolymer, and use of such a copolymer or contact lens material in the production of a contact lens.
The diluent monomer can act as a solvent for the comonomers during copolymerisation to produce the copolymer if no additional solvent is present. Where the diluent monomer and monomer bearing the centre of permanent positive charges are immiscible a solvent can be used to aid mixing.
Particular examples of diluent comonomers include alkyl (alk)acrylate preferably containing 1 to 12, more preferably 1 to 4, carbon atoms in the alkyl group of the ester moiety, such as a methyl (alk)acrylate and butyl (alk)acrylate; a dialkylamino alkyl (alk)acrylate, preferably containing 1 to 4 carbon atoms in each alkyl moiety of the amine and 1 to 4 carbon atoms in the alkylene chain, e.g. 2(dimethylamino)ethyl (alk)acrylate; an alkyl (alk)acrylamide preferably containing 1 to 4 carbon atoms in the alkyl group of the amide moiety; a hydroxyalkyl (alk)acrylate preferably containing from 1 to 4 carbon atoms in the hydroxy moiety, e.g. a 2-hydroxyethyl (alk)acrylate; or a vinyl monomer such as an N-vinyl lactam, preferably containing from 5 to 7 atoms in the lactam ring for instance vinyl pyrrolidone; styrene or a styrene derivative which for example is substituted on the phenyl ring by one or more alkyl groups containing from 1 to 4 carbon atoms, and/or by one or more halogen, such as fluorine atoms.
It is to be understood that throughout the specification (alk)acrylate, (alk)acrylic and (alk)acrylamide mean acrylate or alkacrylate, acrylic or alkacrylic and acrylamide or alkacrylamide respectively. Preferably alkacrylate, alkacrylic and alkacrylamide groups contain from 1 to 4 carbon atoms in the alkyl group thereof and are most preferably methacrylate, methacrylic or methacrylamide groups. Similarly (meth)acrylate, (meth)acrylic and (meth)acrylamide shall be understood to mean acrylate or methacrylate, acrylic or methacrylic and acrylamide or methacrylamide respectively.
Preferably the diluent monomer is selected from vinylpyrrolidone, 2-hydroxyethyl methacrylate, methyl methacrylate and mixtures thereof, most preferably 2-hydroxyethyl methacrylate, methyl methacrylate and mixtures thereof. In one embodiment diluent monomers are vinylpyrrolidone, 2-hydroxyethyl methacrylate and mixtures thereof.
The comonomer bearing the centre of permanent positive charge can either be cationic or zwitterionic. In the latter case the monomer includes within its structure not only a centre of permanent positive charge but also a centre of negative charge. Typically the centre of permanent positive charge in both cationic and zwitterionic comonomers is provided by a quaternary nitrogen atom.
Preferred comonomers which bear a centre of positive charge are of general formula (I)
Yxe2x80x94Bxe2x80x94Xxe2x80x83xe2x80x83(I)
wherein
B is a straight or branched alkylene, oxaalkylene or oligo-oxaalkylene chain or if X contains a carbon-carbon chain between B and the centre of permanent positive charge or if Y contains a terminal carbon atom, a valence bond,
X is a group bearing a centre of permanent positive charge and
Y is an ethylenically unsaturated polymerisable group selected from 
wherein:
R is hydrogen or a C1-C4 alkyl group;
A is xe2x80x94Oxe2x80x94 or xe2x80x94NR1xe2x80x94 where R1 is hydrogen or a C1-C4 alkyl group or R1 is xe2x80x94Bxe2x80x94X where B and X are as defined above.
K is a group xe2x80x94(CH2)pOC(O)xe2x80x94, xe2x80x94(CH2)pC(O)Oxe2x80x94, xe2x80x94(CH2)pOC(O)Oxe2x80x94, xe2x80x94(CH2)pNR2xe2x80x94, xe2x80x94(CH2)pNR2C(O)xe2x80x94, xe2x80x94(CH2)pC(O)NR2xe2x80x94, (CH2)pNR2C(O)Oxe2x80x94, xe2x80x94(CH2)pOC(O)NR2xe2x80x94, xe2x80x94(CH2)pNR2C(O)NR2xe2x80x94(in which the groups R2 are the same or different), xe2x80x94(CH2)pOxe2x80x94, xe2x80x94(CH2)pSO3xe2x80x94, or, optionally in a combination with B, a valence bond, and p is from 1 to 12 and R2 is hydrogen or a C1-C4 alkyl group.
The proviso on whether B may be a valence bond ensures that the centre of permanent positive charge in X is not directly bonded to a heteroatom, such as an oxygen or nitrogen atom in Y.
Preferred monomers which bear a centre of positive charge are those of general formula (II) or (III). 
where R, A, B and X are as defined with reference to formula (I).
Preferably R is hydrogen, methyl, or ethyl, more preferably methyl, so that the monomer of formula (II) is an acrylic acid, methacrylic acid or ethacrylic acid derivative.
In the compounds of formula (III) K may be a valence bond and B a group, K may be a group and B a valence bond, both K and B may be groups or K and B may together be a valence bond. Preferably B is a group where K is a valence bond. Where K is a group then preferably p is from 1 to 6, more preferably 1, 2 or 3 and most preferably p is 1. When K is a group xe2x80x94(CH2)pNR2xe2x80x94, xe2x80x94(CH2)pNR2C(O)xe2x80x94, xe2x80x94(CH2)pC(O)NR2xe2x80x94, xe2x80x94(CH2)pNR2C(O)Oxe2x80x94, xe2x80x94(CH2)pOCNR2xe2x80x94 or xe2x80x94(CH2)pNR2C(O)NR2xe2x80x94 then R2 is preferably hydrogen, methyl or ethyl, more preferably hydrogen.
Preferably B is:
an alkylene group of formula xe2x80x94(CR32)axe2x80x94, wherein the groups xe2x80x94(CR32)xe2x80x94 are the same or different, and in each group xe2x80x94(CR32)xe2x80x94 the groups R3 are the same or different and each group R3 is hydrogen or C1-4 alkyl, preferably hydrogen, and a is from 1 to 12, preferably 1 to 6;
an oxaalkylene group such as alkoxyalkyl having 1 to 6 carbon atoms in each alkyl moiety, more preferably xe2x80x94CH2O(CH2)4xe2x80x94;
an oligo-oxaalkylene group of formula xe2x80x94[(CR42)bO]c(CR42)bxe2x80x94 where the groups xe2x80x94(CR42)xe2x80x94 are the same or different and in each group xe2x80x94(CR42)xe2x80x94 the groups R4 are the same or different and each group R4 is hydrogen or C1-4 alkyl, preferably hydrogen, and b is 2 or 3 and c is from 2 to 11, preferably 2 to 5;
or a valence bond but only if X contains a carbon-carbon chain between B and the centre of positive charge, or if Y contains a terminal carbon atom.
Preferred groups B include a valence bond and alkylene, oxaalkylene and oligo-oxaalkylene groups of up to 12 carbon atoms.
Preferred groups X are the groups of formula (IVA), (IVB), (IVC), (IVD), (IVE) and (IVF) as defined below, of which the groups of formula (IVC) are particularly preferred.
The groups of formula (IVA) are:
xe2x80x94N⊕(R5)3Zxe2x8ax96xe2x80x83xe2x80x83(IVA)
where the groups R5 are the same or different and each is hydrogen or C1-4 alkyl and Z⊕ is a counterion.
Preferably the groups R5 are all the same. It is also preferable that at least one of the groups R5 is methyl, and more preferable that all the groups R5 are methyl.
The counterion Z⊕ present in the compounds of formula (II) or (III) containing a group of formula (IVA) is such that the compounds are neutral salts. The counterion may be exchanged with ions in physiological fluids and thus the specific nature of the counterion is not critical in the present invention. However, physiologically acceptable counterions are preferred. Suitable physiologically acceptable counterions include halide anions, such as chloride, bromide or fluoride ions, other inorganic anions such as sulphate, phosphate and phosphite and organic anions such as aliphatic mono-, di- or tri-carboxylate anions containing from 2 to 25 carbon atoms and optionally bearing one or more hydroxyl groups e.g. acetate, citrate and lactate.
When X is a group of formula (IVA), preferably B is a group of formula xe2x80x94(CR32)xe2x80x94 or xe2x80x94(CR32)2xe2x80x94, eg. xe2x80x94(CH2)xe2x80x94 or xe2x80x94(CH2CH2)xe2x80x94.
The groups of formula (IVB) are: 
where the groups R6 are the same or different and each is hydrogen or C1-4 alkyl and d is from 2 to 4.
Preferably the groups R6 are the same. It is also preferable that at least one of the groups R6 is methyl, and more preferable that the groups R6 are both methyl.
Preferably d is 2 or 3, more preferably 3.
When X is a group of formula (IVB) preferably B is a group of formula xe2x80x94(CR32)xe2x80x94 or xe2x80x94(CR32)2xe2x80x94, eg. xe2x80x94(CH2)xe2x80x94 or xe2x80x94(CH2CH2)xe2x80x94.
The groups of formula (IVC) are: 
where the groups R7 are the same or different and each is hydrogen or C1-4 alkyl, and e is from 1 to 4.
Preferably the groups R7 are the same. It is also preferable that at least one of the groups R7 is methyl, and more preferable that the groups R7 are all methyl.
Preferably e is 2 or 3, more preferably 2.
When X is a group of formula (IVC) preferably B is a group of formula xe2x80x94(CR32)xe2x80x94 or xe2x80x94(CR32)2xe2x80x94, eg. xe2x80x94(CH2)xe2x80x94 or xe2x80x94(CH2CH2)xe2x80x94.
The groups of formula (IVD) are: 
wherein the groups R8 are the same or different and each is hydrogen or C1-4 alkyl, B1 is a valence bond or straight or branched alkylene, oxaalkylene or oligo-oxaalkalkylene group, f is from 1 to 4 and if B is other than a valence bond, Z is 1 and if B is a valence bond Z is 0 if X is directly bonded to an oxygen or nitrogen atom, and otherwise Z is 1.
Preferably the groups R8 are the same. It is also preferable that at least one of the groups R8 is methyl, and more preferable that the groups R8 are all methyl.
Preferably f is 1 or 2, more preferably 2.
Preferably B1 is:
a valence bond;
an alkylene group of formula xe2x80x94(CR3a2)aaxe2x80x94, wherein the groups xe2x80x94(CR3a2)xe2x80x94 are the same or different, and in each group xe2x80x94(CR3a2)xe2x80x94 the groups R3a are the same or different and each group R3a is hydrogen or C1-4 alkyl, preferably hydrogen, and aa is from 1 to 24, preferably 6 to 18;
an oxaalkylene group such as alkoxyalkyl having 1 to 6 carbon atoms in each alkyl moiety, more preferably xe2x80x94CH2O(CH2)4xe2x80x94; or
an oligo-oxaalkylene group of formula xe2x80x94[(CR4a2)baO]caxe2x80x94 where the groups xe2x80x94(CR4a2)xe2x80x94 are the same or different and in each group xe2x80x94(CR4a2)xe2x80x94 the groups R4a are the same or different and each group R4a is hydrogen or C1-4 alkyl, preferably hydrogen, and ba is 2 or 3 and ca is from 1 to 12, preferably 1 to 6.
Preferred groups B1 include a valence bond and alkylene, oxaalkylene and oligo-oxaalkylene groups of up to 24 carbon atoms.
In one embodiment B and B1 are the same.
The groups of formula (IVE) are: 
wherein the groups R9 are the same or different and each is hydrogen or C1-C4 alkyl, B2 is a valence bond or a straight or branched alkylene, oxaalkylene or oligo-oxaalkylene group, g is from 1 to 4 and if B is other than a valence bond, Z is 1 and if B is a valence bond Z is 0 if X is directly bonded to an oxygen or nitrogen atom and otherwise Z is 1.
Preferably the groups R9 are the same. It is also preferable that at least one of the groups R8 is methyl, and more preferable that the groups R8 are all methyl.
Preferably g is 1 or 2, more preferably 2.
Preferably B2 is:
a valence bond;
an alkylene group of formula xe2x80x94(CR3b2)abxe2x80x94, wherein the groups xe2x80x94(CR3b2)xe2x80x94 are the same or different, and in each group xe2x80x94(CR3b2)xe2x80x94 the groups R3b are the same of different and each group R3b is hydrogen or C1-4 alkyl, preferably hydrogen, and ab is from 1 to 24, preferably 6 to 18;
an oxaalkylene group such as alkoxyalkyl having 1 to 6, carbon atoms in each alkyl moiety, more preferably xe2x80x94CH2O(CH2)4xe2x80x94; or
an oligo-oxaalkylene group of formula xe2x80x94[(CR4b2)bbO]cbxe2x80x94 where the groups xe2x80x94(CR4b2)xe2x80x94 are the same or different and in each group xe2x80x94(CR4b2)xe2x80x94 the groups R4b are the same or different and each group R4b is hydrogen or C1-4 alkyl, preferably hydrogen, and bb is 2 to 6 and cb is from 1 to 12, preferably 1 to 6.
Preferred groups B2 include a valence bond and alkylene, oxaalkylene and oligo-oxaalkylene groups of up to 24 carbon atoms.
In one embodiment B and B2 are the same.
The groups of formula (IVF) are: 
wherein the groups R10 are the same or different and each is hydrogen or C1-4 alkyl, B3 is a valence bond or a straight or branched alkylene, oxaalkylene or oligo-oxaalkylene group, h is from 1 to 4 if B is other than a valence bond, Z is 1 and if B is a valence bond Z is 0 if X is directly bonded to an oxygen or nitrogen atom and otherwise Z is 1.
Preferably the groups R10 are the same. It is also preferable that at least one of the groups R10 is methyl, and more preferable that the groups R10 are all methyl.
Preferably h is 1 or 2, more preferably 2.
Preferably B3 is:
a valence bond;
an alkylene group of formula xe2x80x94(CR3c2)acxe2x80x94, wherein the groups xe2x80x94(CR3c2)xe2x80x94 are the same or different, and in each group xe2x80x94(CR3c2)xe2x80x94 the groups R3c are the same or different and each group R3c is hydrogen or C1-4 alkyl, preferably hydrogen, and ac is from 1 to 24, preferably 6 to 18;
an oxaalkylene group such as alkoxyalkyl having 1 to 6 carbon atoms in each alkyl moiety, more preferably xe2x80x94CH2O(CH2)4xe2x80x94; or
an oligo-oxaalkylene group of formula xe2x80x94[(CR4c2)bcO]ccxe2x80x94 where the groups xe2x80x94(CR4c2)xe2x80x94 are the same or different and in each group xe2x80x94(CR4c2)xe2x80x94 the groups R4c are the same or different and each group R4c is hydrogen or C1-4 alkyl, preferably hydrogen, and bc is 2 to 6 and cc is from 1 to 12, preferably 1 to 6.
Preferred groups B3 include a valence bond and alkylene, oxaalkylene and oligo-oxaalkylene groups of up to 24 carbon atoms.
In one embodiment B and B3 are the same.
According to one particular embodiment, the monomer bearing a centre of permanent positive charge is a monomer of formula (V) 
wherein BB is a straight or branched C1-C6 alkylene chain optionally interrupted by one or more oxygen atoms;
nn is from 1 to 12
R11 is H or a C1-C4 alkyl group; and
YY is a group which includes a centre of positive charge. More preferably,
YY is a group selected from:
xe2x80x94⊕N(CH3)3;xe2x80x83xe2x80x83(VIA)

the group BB in (VID) and (VIE) being a linear or branched alkylene chain as defined above and nn being as defined above.
Preferably BB is a group selected from xe2x80x94CH2xe2x80x94, xe2x80x94C(R12)2xe2x80x94, in which R12 is C1-4 alkyl, and xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94.
Preferably in compounds of formula (V), R11 is hydrogen or methyl.
When X is a group as defined under (VID) or (VIE), the group (BB)nn is preferably chosen to avoid steric hindrance in the vicinity of the adjacent xe2x80x94OC(O)xe2x80x94 group, the reactivity of which could be adversely affected by such steric hindrance.
Preferred examples of co-monomers of formula (I) are: 
Particular examples of preferred comonomers bearing a centre of permanent positive charge are 2(methacryloyloxy)ethyl-2xe2x80x2(trimethylammonium)ethyl phosphate inner salt [Compound C above] and 1[4(4xe2x80x2-vinylbenzyloxy)butane]- 2xe2x80x3(trimethylammonium)ethyl phosphate inner salt [a compound of formula (III)].
Comonomers bearing a centre of permanent positive charge, such as those of formulae (II) and (III), and comonomers of formula (V) may be prepared by conventional techniques using known reactions, for example using a suitable substituted alkyl (alk)acrylate, glycerophosphoryl choline or suitable substituted styrene as starting material.
Examples of suitable substituted alkyl (alk)acrylates include dimethylaminoethyl(meth)acrylate and 2-hydroxyethyl(meth)acrylate.
Comonomers of formula (II) or (III) containing a group of formula (IVA), (IVB) or (IVC) and comonomers of formula (V) including a group of formula (VIA), (VIB), and (VIC) may be prepared as described in Reference Examples 1 to 4 or by analogous known methods.
Comonomers of formula (II) or (III) containing a group of formula (IVD) and comonomer of formula (V) including a group of formula (VID) may be prepared by selective acylation of glycerophosphorylcholine or analogues thereof at the primary hydroxyl group with an activated acid derivative such as an acid anhydride O[C(O)B1CH3]2 or an acid halide CH3B1COHal where B1 is as defined above and Hal is halogen, followed by acylation of the secondary hydroxyl group with an appropriate acylating agent, for example methacryloyl chloride. Purification, for example by column chromatography on a suitable support, may be performed after each acylation or after the second acylation only. Suitable activated acid derivatives include acid anhydrides, acid halides, reactive esters and imidazolides. The acylations may be performed in a suitable anhydrous, aprotic solvent, for example N,N-dimethylformamide, optionally in the presence of a suitable non-nucleophilic base, for example triethylamine.
Alternatively, the primary alcohol group in glycerophosphoryl choline or an analogue thereof may be blocked by reaction with a suitable protecting group reagent, for example t-butyldimethylsilyl chloride, under standard conditions and the secondary hydroxy group then treated with an acylating agent such as methacryloyl chloride. The t-butyldimethylsilyl protecting group may be removed by treatment with a dilute organic or mineral acid, for example p-toluene sulphonic acid, hydrochloric acid or with tetra-butylammonium fluoride. The deblocked primary hydroxyl group may then be treated with an activated acid derivative such as an acid anhydride O[C(O)B1CH3]2 or acid halide CH3B1COHal where B1 is as defined above, and Hal is halogen.
Analogues of glycerophosphorylcholine may be prepared by reaction of phosphorus oxychloride with a bromoalcohol in an inert aprotic solvent, such as dichloromethane, to give a bromoalkylphosphorodichloridate. The dichloro derivative thus produced may then be treated with 2,2-dimethyl 1,3-dioxolane-4-methanol in the presence of a base, for example triethylamine, followed by acid hydrolysis to give a bromoalkylphosphorogylcerol derivative. This may then be treated with an amine NR83, where R8 is as defined above, for example trimethylamine, to generate the glycerophosphorylcholine analogue. This preparation is depicted in the following scheme. 
where R8 and f are as defined in relation to groups of formula (IVD).
Comonomers of formula (II) or (III) containing a group of formula (IVE) and comomers of formula (V) containing a group of formula (VIE) may be prepared by the selective acylation of glycerophosphorylcholine or an analogue thereof at the primary hydroxyl group with for example, methacryloyl chloride followed by reaction at the secondary hydroxyl group using an activated acid derivative, such as an acid halide O[C(O)B2CH3]2 or an acid halide CH3B2COHal, where B2 is as defined above and Hal is halogen. The intermediates and final products may be purified, as necessary using column chromatography. Optionally, protecting group strategy, similar to that outlined above in relation to production of comonomers containing a group of formula (IVD), may be employed.
Comonomers of formula (II) or (III) containing a group of formula (IVF) may be prepared in an analogous manner to comonomers containing groups of formula (IVD) or (IVE).
The copolymers of the invention also comprise residues of difunctional and/or trifunctional comonomers. Such comonomers are capable of crosslinking the polymer during polymerisation. Conventional crosslinking agents may be used.
Examples of suitable crosslinking comonomers include alkane diol or triol di- or tri(alk)acrylates, eg (meth)acrylates, preferably containing 1 to 8 carbon atoms in the diol or triol residue; alkylene di- or tri(alk)acrylamides, e.g. (meth)acrylamides, preferably containing 1 to 6 carbon atoms in the alkylene group and and di- or tri-vinyl compounds such as di- or tri-vinyl benzene compounds. Particular examples of crosslinking agents include ethyleneglycoldimethacrylate, tetraethyleneglycol dimethacrylate, trimethylolpropanetrimethacrylate and N,N-methylenebisacrylamide.
Optionally the comonomer mixture used for polymerising the copolymer further comprises a gel swelling monomer such as an N-vinyl lactam, methacrylic acid or acrylic acid and where appropriate a bulking or solvating agent such as a solvent, for example, an alcohol or water.
Polymers of the invention may be prepared by copolymerising monomers bearing a centre of permanent positive change, diluent monomers and crosslinking monomers usually by bulk polymerisation in an appropriate mould. Additionally a solvent or solvent mixture may be included to provide a suitable reaction medium for immiscible comonomers. Suitable solvents include water, halogenated organic solvents and non-halogenated organic solvents. Initiators and/or reagents to modify the bulk morphology of the final polymer may also be included. Any conventional technique may be used for the polymerisation, typically thermal polymerisation or ultraviolet polymerisation.
The invention therefore further provides a method of preparing a crosslinked polymer which comprises copolymerising a monomer composition, such as a monomer solution, comprising a diluent monomer or monomers, a comonomer or comonomers including within its structure a centre of permanent positive charge, and a monomoner or monomers which will crosslink the resultant polymer. Optionally, the monomer composition further comprises a solvent or solvent mixture and a polymerisation initiator or initiators.
The monomer composition which is subjected to polymerisation typically comprises at least 30%, preferably at least 60%, and up to 99.79% by weight of diluent monomer. It typically comprises at least 0.2% and up to 50% monomer or monomers which contain a centre of permanent positive charge and from 0.01% to 20% by weight of crosslinking monomer. Optionally up to 10% by weight of gel swelling monomer is included.
In one embodiment the monomer composition which is subjected to polymerisation typically comprises at least 70%, preferably at least 80% by weight of the diluent monomer. It further comprises at least 0.2% and up to 20% of monomer or monomers which bear a centre of permanent positive charge and, optionally, up to 10% by weight of gel-swelling monomer or monomers.
The monomer composition may comprise conventional further polymer ingredients such as cross-linking agents and polymerisation initiators. These further ingredients are in one embodiment used in a total amount from 0.1 to 5%, typically from 0.2 to 3% and preferably about 0.5% by weight relative to the weight of the monomer composition prior to polymerisation.
Preferably the monomer composition comprises at least 0.01% and up to 10% of crosslinking monomer or monomers.
Examples of suitable initiators include bis(4-tertiarybutylcyclohexyl)-peroxydicarbonate, benzoylperoxide, 2,2xe2x80x2-azo-bis(2-methylpropionitrile) [i.e. azo-bis-isobutyro nitrile], 1-benzyl-2-hydroxy-2-dimethylethane-1-one and benzoin methylether. An initiator is generally used in a total amount from 0.1% to 5, typically from 0.2% to 3% and preferably about 0.5% by weight relative to the weight of the total monomer composition prior to polymerisation.
Additionally the monomer composition may have added to it a solvent or solvent mixture. Examples of suitable solvents are ethanol, methanol and water. When present, solvent suitably comprises from 0.1 to 50 weight % of the total reaction mixture, preferably from 5 to 40 weight %.
The polymer is prepared by dissolving the monomer or monomers bearing the centre of positive charge in the diluent monomer or monomers or diluent monomer/solvent mixture together with the crosslinking monomer or monomers and if present the polymerisation initiator or initiators. The solution thus formed is then purged with nitrogen, to remove any oxygen which may be present before the polymerisation process is begun. Polymerisation is carried out in a sheet-forming mould, a contact lens precursor button (thick round disc) mould, a contact lens mould or to provide a cylindrical polymer rod. For example, when carried out in a sheet-forming mould the monomer solution may be injected between two spaced plates and then polymerised in situ to generate a polymer sheet.
Generally the copolymers of the invention will be produced by copolymerisation in the absence water. This produces a xerogel material which can be moulded into contact lenses directly or moulded to give contact lens buttons which can be lathe cut using methods known in the art to produce contact lenses. The xerogel material may be washed in water or in aqueous buffer to remove any excess monomer and initiator. The xerogel material can be subsequently hydrated to produce hydrogel with an equilibrium water content of up to 90%, and preferably from 30 to 80%.
The polymers of the invention are both transparent and water swellable and therefore suitable for use as contact lens materials. In particular, the polymer may be suitable for use in contact lenses which are for example soft or gas permeable contact lenses.
The invention further provides contact lenses made from polymers of the invention as hereinbefore defined.